1. Field of the Invention
The present invention relates to digital communications equipment such as digital radio transceivers. In particular, the invention relates to spread spectrum digital radio transceivers and digital radio transceivers using differentially encoded four state modulation.
2. State of the Art
In recent years, the use of wireless local area networks ("LANs") has become widespread. In wireless LANs, digital radio transceivers are used to link together various computers, which may be mobile or stationary. In 1985, the FCC established regulations to allow unlicensed use of certain bands if spread-spectrum techniques are used. In spread spectrum transmission, the energy radiated during radio transmission is spread across a wide spectrum of frequencies and is therefore less liable to cause substantial interference with other radio communications. FCC spread-spectrum regulations allow greater transmission power to be used without requiring special licensing, increasing the attainable range of communications for unlicensed systems.
There exist two principle spread spectrum transmission techniques, direct sequencing and frequency hopping. In direct sequencing, spreading is achieved through multiplication of the data by a binary pseudo random sequence whose chipping rate is many times the data rate. In frequency hopping, the carrier frequency remains at a given frequency for a duration and then hops to a new frequency somewhere in the spreading bandwidth.
Direct sequencing allows for coherent demodulation. In coherent demodulation, the receiver exploits knowledge of the carrier wave's phase reference to detect the signals. With frequency hopping, however, phase coherence is difficult to maintain; hence it is usually demodulated noncoherently. Non-coherent demodulation refers to demodulation performed with no knowledge of phase, i.e., without phase estimation processing. Noncoherent demodulation results in the advantage of reduced complexity over coherent demodulation but at the cost of an increased probability of error.
Frequency hopping offers other advantages with respect to direct sequence. Frequency hopping enables higher rates to be achieved without requiring very high speed logic that an equivalent direct sequency system would require. Frequency diversity, a technique used to combat multipath fading by transmitting data in multiple frequencies and thus increasing the likelihood that the data will make it through the channel uncorrupted, can be achieved at no additional cost.
As data files become increasingly large, support of a high data rate becomes an increasingly important factor in digital communications. High data rates, however, require larger bandwidths. The FCC, besides regulating transmission power, has also issued spectrum occupancy requirements. In the case of frequency hopping radio transceivers, more than 90% of the transmission energy must occur within a 1 megahertz bandwidth defined by the center frequency, f.sub.c, .+-.500 kHz. This spectrum occupancy requirement defines what may be referred to as the "transmission mask".
At lower data rates, relatively simple modulations schemes may be used while fitting within a given transmission mask. One such modulation scheme is binary frequency shift keying (BFSK). BFSK has been used in direct sequence spread spectrum radio transceivers to achieve data rates of up to several hundred kilobits per second (kbps). For data rates in the megabits per second range, the use of BFSK results in a signal that does not fit within the allowed bandwidth or in an excessively costly design not suitable for cost-sensitive applications. A more complex modulation scheme is therefore required.
An example of one such modulation scheme is differential quadrature phase shift keying ("DQPSK"). In DQPSK, four information states are defined by changing the phase of a carrier signal in 90.degree. increments. DQPSK is therefore a four state modulation scheme, allowing twice the data rate to be achieved through the same channel as compared to two state modulation. In DQPSK, the carrier phase of the previous signaling interval is used as a phase reference for demodulation. The information is therefore carried by the difference in phase between two successive waveforms. As compared to nondifferential QPSK in which the received signal is compared with the carrier reference, in DQPSK, two noisy signals are in effect compared with each other. Hence, DQPSK exhibits greater noise but allows for reduced system complexity. DQPSK can be demodulated using so-called "differentially coherent" demodulation, which does not require traditional coherent demodulation techniques but still requires more complex (IQ) demodulation. In addition, DQPSK is a type of phase modulation and as such is vulnerable to phase inversions that are commonly encountered in environments with multipath propagation such as the indoor environment.
Frequency shift keying (FSK) modulation schemes are generally less costly to implement than phase shift keying (PSK) modulation schemes. Although DQPSK performs better under lower signal-to-noise conditions, it is unsuitable for frequency hopping due to the need to know the carrier phase. In frequency hopping systems the carrier is always changing frequency, which creates problems in DQPSK systems, since small frequency errors translate into large phase errors. In addition, operation in multipath indoor environments creates phase reversals of the carrier as a transceiver moves across a null. Phase Shift Keying systems have difficulty dealing with this situation.
What is needed then is a digital communications system that achieves a spectral efficiency equivalent to DQPSK or QPSK but does not depend on knowledge of the carrier's phase. Frequency Shift Keying has been used traditionally for frequency hopping systems but does not achieve the desired spectral efficiency. The present invention addresses this issue.